This invention relates to a two stage pressure swing adsorption (PSA) process for producing high purity gas from a mixture of a plurality of gases and more particularly, to a PSA process for producing high purity oxygen from air.
Conventional PSA processes for generating oxygen from an air stream commonly use a fixed bed of adsorbent material adapted to adsorb nitrogen from air, such as zeolite, so that an oxygen-rich product gas exits the bed. The principles of separation involved in such an adsorption system are based upon equilibrium separation, i.e., upon the adsorbent material""s ability to hold nitrogen more strongly than oxygen. Present-day synthetic zeolites used in PSA processes are capable of achieving virtually a complete separation between nitrogen and oxygen. However, the adsorption isotherms of oxygen and argon on these materials are almost identical and a passage of feed air through a zeolite bed has no significant effect on the ratio of oxygen to argon which is typically about 21:1. Thus, the percentage by volume of argon in the oxygen-rich product stream, assuming that all of the nitrogen is adsorbed by the zeolite, is about 5 percent. Therefore, PSA processes which employ nitrogen equilibrium selective materials cannot normally generate a product stream containing an oxygen concentration which is appreciably greater than 95.0 percent.
Materials which preferentially adsorb oxygen can also be employed in PSA processes for producing oxygen from an air stream. In such a process, the oxygen-rich product is collected from the adsorbent bed during the regeneration step of each cycle. At the present time the most commonly used oxygen selective adsorbent materials are carbon molecular sieves (CMS). The separation achieved with CMS is a result of the material""s more rapid adsorption of oxygen than of nitrogenxe2x80x94which is known as kinetic selectivity. From the point of view of oxygen/nitrogen separation, the kinetic selectivity of CMS is significantly less efficient than the equilibrium selectivity of zeolite. Further, the oxygen product obtained from an air feed, using CMS as an adsorbent material, contains a considerable portion of unseparated nitrogen.
In practice the rates of adsorption of nitrogen and argon on CMS are about the same so that in the case of an air feed, the balance of the oxygen product will contain nitrogen and argon approximately in their atmospheric ratio 78:1.
In summary, PSA processes for production of oxygen from air which use nitrogen equilibrium selective adsorbents can give maximum oxygen purity of about 95.0%, with the balance represented virtually entirely by argon. PSA processes for production of oxygen from air, which use CMS as the adsorbent, can give a maximum oxygen purity of about 80%, with the balance represented by nitrogen and argon in their atmospheric ratio, i.e., about 19.75% nitrogen and 0.25% argon.
However, oxygen of a purity greater than 95.0% is needed in welding and cutting processes as well as in some medically-related applications. Accordingly, it is desirable to provide a PSA process capable of generating a product stream containing an oxygen concentration which is greater than 95.0 percent from an air feed stream.
Several PSA systems are known in the prior art which can produce a product stream containing an oxygen concentration which is greater. than 95.0% from an air feed stream. All such systems utilize a two stage PSA arrangement, i.e., there are two distinct mass transfer zones in the PSA process.
One group of two stage PSA processes for production of high purity oxygen from feed air is represented by U.S. Pat. No. 4,190,424 (Armond et al.), U.S. Pat. No. 4,959,083 (Garrett), U.S. Pat. No. 4,973,339 (Bansal) and by publications by Seemann et al. (Chem. Eng. Technol. Vol 11, p 341, 1988) and Hayashi et. al. (Gas Sep. Purif. Vol 10 No. 1, p 19, 1996). The first stage employs one or several beds of a CMS which adsorbs oxygen more rapidly, as compared to nitrogen and argon (i.e., an oxygen kinetically-selective material). A feed stream of air constituents (i.e., oxygen, nitrogen, and argon) is delivered to the first stage where oxygen is adsorbed at a higher rate than nitrogen and argon. The adsorbed oxygen is subsequently desorbed and is fed to a second stage which uses one or several beds of zeolite that adsorbs nitrogen preferentially to oxygen and argon (nitrogen equilibrium-selective material). High purity oxygen is collected at the exit of the zeolite bed.
The key to the high purity oxygen product obtained from this PSA process is not just the ability of the first CMS stage to provide an oxygen-enriched feed to the second nitrogen adsorbing zeolite stage. More particularly, it is the ability of the CMS stage to provide a feed which is depleted in argon, the one major constituent of atmospheric air which a zeolite is incapable of separating from oxygen.
Another group of two stage PSA processes for production of high purity oxygen from feed air is represented by U.S. Pat. No. 5,395,427 (Kumar et al.), U.S. Pat. No. 5,137,549 (Stanford et al.) and U.S. Pat. No. 4,190,424 (Armond et al.). The first stage comprises two beds of zeolite and separates nitrogen, carbon dioxide and water vapor from atmospheric air and passes oxygen, argon and residual nitrogen to the second stage. The second stage includes a pair of beds with oxygen selective material that adsorb oxygen and pass the argon and the residual nitrogen. The high purity oxygen product is recovered upon depressurization of the second stage.
The high purity of the oxygen product is achieved by rinsing the oxygen selective adsorbent with high purity oxygen prior to the depressurization step.
Another two stage PSA process for production of high purity oxygen from feed air is disclosed in U.S. Pat. No. 4,959,083 (Garrett). The first stage comprises a bed of CMS which adsorbs oxygen more rapidly than nitrogen. The adsorbed oxygen is desorbed from the first stage and flows to a second stage which comprises another bed of CMS. The adsorbed oxygen in the second stage is subsequently desorbed and collected as high purity oxygen product.
Another group of two stage PSA processes for production of high purity oxygen from feed air is represented by U.S. Pat. No. 5,226,933 (Knaebel et al.) and U.S. Pat. No. 5,470,378 (Kandybin et al.). A first stage utilizes nitrogen equilibrium-selective adsorbent (zeolite) while the second stage utilizes an argon equilibrium selective adsorbent (silver mordenite). The adsorbents can be placed in separate beds or in a single bed. When the feed air is introduced into the system, nitrogen is removed in the first stage, argon is removed in the second stage, and high purity oxygen is collected at the exit of the system as product.
There are a number of drawbacks in the prior art PSA processes for producing high purity oxygen from an air feed.
1. In the prior art there is an incompatibility between the stage cycle times when one of the stages utilizes an equilibrium selective adsorbent such as zeolite and the other stage utilizes a kinetically selective adsorbent such as CMS. This leads to an asynchronous mode of operation of the stages and complicates the PSA cycle. In addition, a buffer tank must be placed between the stages.
2. The mode of operation of a CMS requires relatively high adsorption pressuresxe2x80x94typically between 6 atm and 10 atm. For silver mordenite the required adsorption pressures are even higherxe2x80x94between 10 atm and 20 atm. Thus such prior art PSA systems are characterized by high energy consumption.
3. The prior art PSA systems which use an oxygen selective adsorbent in the second stage always employ an oxygen rinse prior to the depressurization in order to achieve high purity of the oxygen product. This reduces the productivity of the PSA system because high purity oxygen product is used as the rinse gas. Also, power requirements increase because the high purity oxygen product is obtained at low pressure during depressurization and at least a portion of the high purity oxygen product must be recompressed again to the high adsorption pressure to supply the cocurrent (with respect to the feed) high pressure purging gas.
4. The prior art PSA processes which use an oxygen selective adsorbent in the second stage rely on use of oxygen enriched streams from the second stage oxygen selective beds for regeneration of the first stage nitrogen selective beds, resulting in a decrease in the productivity of the second stage beds.
Accordingly, it is an object of the invention to provide an improved dual stage PSA process for the production of high purity oxygen, wherein only equilibrium selective adsorbents are employed and the operation of the stages is synchronized.
It is another object of the invention to provide an improved dual stage PSA process for the production of high purity oxygen, which employs modest adsorption pressures and thus exhibits reduced power requirements.
It is a further object of the invention to provide an improved dual stage PSA process for the production of high purity oxygen, which avoids the need for use of an oxygen rinse step.
It is a further object of the invention to provide an improved dual stage PSA process for the production of high purity oxygen, which enables recovery as product, all of the high purity oxygen effluent of the second stage bed, thereby increasing the productivity of the second stage.
The present invention is a two stage PSA process for producing high purity oxygen from a feed air stream. Water, carbon dioxide and nitrogen are removed in a first stage. An oxygen selective adsorbent is used to adsorb oxygen in the second stage. High purity oxygen product is recovered during regeneration of the second stage. Importantly, the high purity of the oxygen product is achieved without inclusion of an oxygen rinse step in the process cycle. The high purity oxygen product is obtained by collecting the middle cut of the second stage effluent stream during regeneration.
In brief, the method of the invention:
i) produces high purity ( greater than 95.5% ) oxygen using oxygen equilibrium selective adsorbent;
ii) uses no high pressure rinse step (cocurrent displacement step) in the PSA cycle;
iii) enables upper and lower stages to be regenerated independently and avoids interaction between the stages during regeneration; and
iv) operates the stages in synchronism using the same step times, consequently, avoiding need for buffer tank(s) between the stages.